Conventionally, in the manufacturing process of electronic industrial products such as a semiconductor device, a liquid crystal device and the like, wet cleaning has been performed as an important step of removing fine particles adhering onto a substrate surface. A general method of removing the fine particles is the RCA cleaning method using washing water made by mixing ammonia water, hydrogen peroxide solution and pure water at a volume ratio of, for example, 1:1:5. In the RCA washing method, it is also performed to rinse the washed substrate surface, using ultrapure water.
However, in this method, large amounts of the ammonia water and hydrogen peroxide solution are used. So the large amount of waste liquid such as the used ammonia water and hydrogen peroxide solution and the large amount of ultrapure water for the rinse are wasted. This increases the load of waste liquid disposal. Therefore, this method causes a large environment load problem.
Hence, in recent years, in the above-described manufacturing process of electronic industrial products, a cleaning method using, as the washing water, ultrapure water in which hydrogen gas is dissolved (hereinafter called as hydrogen water) is just beginning to be used. From the ultrapure water in which the hydrogen gas is dissolved, impurities such as dissolved ions, fine particles, organic substances and the like have been removed to the limit. Therefore, static electricity sometimes occurs in the ultrapure water due to friction between the washing water and an inner wall of a pipe and nozzle or the like. In this case, current flows when the washing water comes into contact with electronic components such as the silicon wafer which is a body to be washed, and thereby breaks semiconductor elements on the silicon wafer, resulting in lower yield of products. Therefore, ammonia or the like is dissolved in the washing hydrogen water to adjust the specific resistivity of the washing water to a predetermined value, thereby imparting an antistatic ability to the washing water.
In the method of adding ammonia to the hydrogen water and using it for washing the semiconductor substrate, for example, a liquid concentrate of ammonia water is introduced from a liquid concentrate tank in which the liquid concentrate of ammonia water is accommodated, to a diluting tank. Here, the liquid concentrate of ammonia water is diluted with ultrapure water to generate diluted ammonia water. Thereafter, the diluted ammonia water is added to hydrogen water to obtain washing hydrogen water.
However, the present inventors have found that when the washing hydrogen water is supplied to a washing apparatus for the silicon wafer and the washing of the silicon wafer is repeated in a single-wafer mode of washing the silicon wafer one by one while adding the diluted ammonia water to the hydrogen water by this method, debris of a fine particle form is generated on the washed substrate surface after a lapse of a long time from the start of adjustment of the diluted ammonia water.